The Anglo-Australian Telescope (AAT) is a 3.9 m equatorially mounted

Equatorial mount

An equatorial mount is a mount for instruments that follows the rotation of the sky by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras...

 telescope operated by the Australian Astronomical Observatory and situated at the Siding Spring Observatory

Siding Spring Observatory

Siding Spring Observatory near Coonabarabran, New South Wales, Australia, part of the Research School of Astronomy & Astrophysics at the Australian National University , incorporates the Anglo-Australian Telescope along with a collection of other telescopes owned by the Australian National...

, Australia

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

 at an altitude of a little over 1100 m. According to a Sydney Morning Herald article, in 2009 the telescope was ranked 5th out of the world's optical telescopes, and was considered the most scientifically productive optical telescope in the world.